34 results for Alloway, Brent

  • Revision of the marine chronology in the Wanganui Basin, New Zealand, based on the isothermal plateau fission-track dating of tephra horizons

    Alloway, Brent; Pillans, BJ; Sandhu, AS; Westgate, JA (1993-01)

    Journal article
    The University of Auckland Library

    The occurrence of tephra horizons in basins adjacent to volcanic arcs provide an excellent opportunity for establishing a reliable chronostratigraphic framework for detailed sedimentological studies. In this study, three widespread and stratigraphically important rhyolitic tephra horizons interbedded in Plio/Pleistocene strata of the Wanganui Basin, New Zealand, are dated by application of the isothermal plateau fission-track (ITPFT) technique to hydrated glass shards. All glass samples were corrected for annealing and consequently yield reliable ages. Rangitawa Tephra yielded statistically indistinguishable ages from three localities that are in excellent agreement with recently determined zircon fission-track age estimates of ca. 0.35 Ma. ITPFT ages of 1.05 ?? 0.05 and 1.63 ?? 0.15 Ma for Potaka Pumice and Pakihikura Pumice, respectively, are considerably older than previous FT estimates but consistent with new magnetostratigraphic data that places the Potaka within the Jaramillo Subchron, and Pakihikura within the Matuyama Chron between the Cobb Mountain and Olduvai Subchrons. Combining our fission-track ages with the magnetostratigraphy, the true age of sediments within the Wanganui Basin is found to be significantly underestimated. Sedimentation rates of between ca. 680-630 m/Ma from 1.63 Ma to 0.35 Ma are calculated in the eastern part of the basin and are much lower than those calculated using the previous FT chronology. This new ITPFT-age data demonstrates that the existing Plio/Pleistocene marine chronology in New Zealand will require age revision and has important implications when considering the evolution of several other sedimentary basins in southern North Island that contain the same ITPFT-dated tephra horizons.

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  • 10,000 Years of explosive eruptions of Merapi Volcano, Central Java: archaeological and modern implications

    Newhall, CG; Bronto, S; Alloway, Brent; Banks, NG; Bahar, I; del Marmol, MA; Hadisantono, RD; Holcomb, RT; McGeehin, J; Miksic, JN; Rubin, M; Sayudi, SD; Sukhyar, R; Andreastuti, MD; Tilling, RI; Torley, R; Trimble, D; Wirakusumah, AD (2000-07)

    Journal article
    The University of Auckland Library

    Stratigraphy and radiocarbon dating of pyroclastic deposits at Merapi Volcano, Central Java, reveals ???10,000 years of explosive eruptions. Highlights include: (1) Construction of an Old Merapi stratovolcano to the height of the present cone or slightly higher. Our oldest age for an explosive eruption is 9630??60 14C y B.P.; construction of Old Merapi certainly began earlier. (2) Collapse(s) of Old Merapi that left a somma rim high on its eastern slope and sent one or more debris avalanche(s) down its southern and western flanks. Impoundment of Kali Progo to form an early Lake Borobudur at ???3400 14C y B.P. hints at a possible early collapse of Merapi. The latest somma-forming collapse occurred ???1900 14C y B.P. The current cone, New Merapi, began to grow soon thereafter. (3) Several large and many small Buddhist and Hindu temples were constructed in Central Java between 732 and ???900 A.D. (roughly, 1400???1000 14C y B.P.). Explosive Merapi eruptions occurred before, during and after temple construction. Some temples were destroyed and (or) buried soon after their construction, and we suspect that this destruction contributed to an abrupt shift of power and organized society to East Java in 928 A.D. Other temples sites, though, were occupied by ???caretakers??? for several centuries longer. (4) A partial collapse of New Merapi occurred s history, most recently during the 19th century. Are the relatively small eruptions of the 20th century a new style of open-vent, less hazardous activity that will persist for the foreseeable future? Or, alternatively, are they merely low-level ???background??? activity that could be interrupted upon relatively short notice by much larger explosive eruptions? The geologic record suggests the latter, which would place several hundred thousand people at risk. We know of no reliable method to forecast when an explosive eruption will interrupt the present interval of low-level activity. This conclusion has important implications for hazard evaluation.

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  • The role of Egmont-sourced tephra in evaluating the paleoclimatic correspondence between the bio- and soil-stratigraphic records of central Taranaki, New Zealand

    Alloway, Brent; McGlone, MS; Neall, VE; Vucetich, CG (1992)

    Journal article
    The University of Auckland Library

    On the lower eastern flanks of Egmont Volcano, western North Island, New Zealand, several Egmont-sourced tephras have been correlated to a radiocarbon dated peat site that provides a late last-glacial to early post-glacial record of vegetation and climate change in central Taranaki. The pollen spectra at this site indicates a mosaic of grassland-shrubland and Prumnopitys taxifolia-dominant forest at ca. 13,100 BP and suggests a climate cooler and drier than at present. The ominance of tall podocarp forest by ca. 12,900 BP suggests that climatic amelioration was rapid towards the end of the last glacial period. Initially this forest was dominated by Prumnopitys taxifolia but at ca. 11,000 BP was succeeded by Dacrydium cupressinum indicating a steady shift to warmer and moister climate. Tephra interbedded with peat allows precise chrono-correlation into the Andisol-forming environment. The rapidity of climatic amelioration identified from variations in Andisol morphology and mineralogy concurs with that indicated from pollen changes in the peat.

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  • Particle size analyses of Late Quaternary allophane-dominated andesitic deposits from New Zealand

    Alloway, Brent; Neall, VE; Vucetich, CG (1992)

    Journal article
    The University of Auckland Library

    On the western North Island, New Zealand, a Late Quaternary sequence of allophane-dominated cover-bed (Andisol) deposits have accumulated from intermittent accretion and rapid, subsequent weathering of aerially transported detritus of dominantly andesitic provenance. Particle size analyses of Andisol samples were attempted for textural classification and provenance studies. The hydrometer and sedigraph techniques were unsuccessful due to difficulties arising from the flocculation of short-range order clay and organic constituents (SROCO), which prevented complete particle dispersion. Neither acidic (HCl) nor alkaline (NH4OH or NaOH) solutions were effective in completely dispersing samples, so an alternative chemical procedure was devised. This alternative pretreatment involves the selective dissolution of Andisol SROCO constituents by 0.2 mol acid-oxalate reagent (pH 3.0???3.5), and has considerable potential in the determination of particle size, soil textural classification and provenance of allophane-dominated andesitic deposits.

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  • Radiocarbon constraints on the extent and evolution of the South Pacific glacial carbon pool

    Ronge, TA; Tiedemann, R; Lamy, F; K??hler, P; Alloway, Brent; De Pol-Holz, R; Pahnke, K; Southon, J; Wacker, L (2016-05-09)

    Journal article
    The University of Auckland Library

    During the last deglaciation, the opposing patterns of atmospheric CO2 and radiocarbon activities (??14C) suggest the release of 14C-depleted CO2 from old carbon reservoirs. Although evidences point to the deep Pacific as a major reservoir of this 14C-depleted carbon, its extent and evolution still need to be constrained. Here we use sediment cores retrieved along a South Pacific transect to reconstruct the spatio-temporal evolution of ??14C over the last 30,000 years. In ???2,500???3,600???m water depth, we find 14C-depleted deep waters with a maximum glacial offset to atmospheric 14C (????14C=???1,000???). Using a box model, we test the hypothesis that these low values might have been caused by an interaction of aging and hydrothermal CO2 influx. We observe a rejuvenation of circumpolar deep waters synchronous and potentially contributing to the initial deglacial rise in atmospheric CO2. These findings constrain parts of the glacial carbon pool to the deep South Pacific.

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  • Climate of the last glaciation in New Zealand, based on aerosolic quartz influx in an andesitic terrain

    Alloway, Brent; Stewart, RB; Neall, VE; Vucetich, CG (1992-09)

    Journal article
    The University of Auckland Library

    On western North Island, New Zealand, a record of climatic change during the last glaciation is preserved in a terrestrial coverbed sequence of dominantly andesitic provenance. Here, a succession of five loess-like Andisol units postdates the global high sea-level stand of oxygen isotope substage 5e (<125,000 yr). Tephra erupted from western and central North Island allow precise chronological correlation of the loess-like units. Aerosolic quartz additions determined by quantitative X-ray diffraction (XRD) record two major peaks that correlate with oxygen isotope stages 2 and 4. The most likely source of quartz-rich dust at these times is the surrounding continental shelf, then exposed by low sea level; however, quartz of Australian provenance may also be represented. This study provides the first confirmation from the terrestrial New Zealand record that rates of atmospherically transported particles increase during glacial stages.

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  • Onshore???offshore correlation of Pleistocene rhyolitic eruptions from New Zealand: implications for TVZ eruptive history and paleoenvironmental construction

    Alloway, Brent; Pillans, BJ; Carter, L; Naish, TR; Westgate, JA (2005-08)

    Journal article
    The University of Auckland Library

    Taupo Volcanic Zone (TVZ), in the North Island, New Zealand, is arguably the most active Quaternary rhyolitic system in the world. Numerous and widespread rhyolitic tephra layers, sourced from the TVZ, form valuable chronostratigraphic markers in onshore and offshore sedimentary sequences. In deep-sea cores from Ocean Drilling Program (ODP) Leg 181 Sites 1125, 1124, 1123 and 1122, located east of New Zealand, ca 100 tephra beds are recognised post-dating the Plio-Pleistocene boundary at 1.81 Ma. These tephras have been dated by a combination of magnetostratigraphy, orbitally tuned stable-isotope data and isothermal plateau fission track ages. The widespread occurrence of ash offshore to the east of New Zealand is favoured by the small size of New Zealand, the explosivity of the mainly plinian and ignimbritic eruptions and the prevailing westerly wind field. Although some tephras can be directly attributed to known TVZ eruptions, there are many more tephras represented within ODP-cores that have yet to be recognised in near-source on-land sequences. This is due to proximal source area erosion and/or deep burial as well as the adverse effect of vapour phase alteration and devitrification within near-source welded ignimbrites. Despite these difficulties, a number of key deep-sea tephras can be reliably correlated to equivalent-aged tephra exposed in uplifted marine back-arc successions of Wanganui Basin where an excellent chronology has been developed based on magnetostratigraphy, orbitally calibrated sedimentary cycles and isothermal plateau fission track ages on tephra. Significant Pleistocene tephra markers include: the Kawakawa, Omataroa, Rangitawa/Onepuhi, Kaukatea, Kidnappers-B, Potaka, Unit D/Ahuroa, Ongatiti, Rewa, Sub-Rewa, Pakihikura, Ototoka and Table Flat Tephras. Six other tephra layers are correlated between ODP-core sites but have yet to be recognised within onshore records. The identification of Pleistocene TVZ-sourced tephras within the ODP record, and their correlation to Wanganui Basin and other onshore sites is a significant advance as it provides: (1) an even more detailed history of the TVZ than can be currently achieved from the near-source record, (2) a high-resolution tephrochronologic framework for future onshore-offshore paleoenvironmental reconstructions, and (3) well-dated tephra beds correlated from the offshore ODP sites with astronomically tuned timescales provide an opportunity to critically evaluate the chronostratigraphic framework for onshore Plio-Pleistocene sedimentary sequences (e.g. Wanganui Basin, cf. Naish et al. [1998. Quaternary Science Reviews 17 695???710].

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  • Integrated outcrop, drill core, borehole and seismic stratigraphic architecture of a cyclothemic, shallow???marine depositional system, Wanganui Basin, New Zealand

    Naish, TR; Field, BD; Zhu, H; Melhuish, A; Carter, RM; Abbott, ST; Edwards, S; Alloway, Brent; Wilson, GS; Niessen, F; Barker, A; Browne, GH; Maslen, G (2005)

    Journal article
    The University of Auckland Library

    Late Pliocene to mid???Pleistocene (c. 2.1???0.4 Ma) strata exposed in the, now classical, Nukumaru and Castlecliff coastal cliff sections north???west of Wanganui comprise 25, 6th (41 ka) order and 5th (100 ka) order, shallow???marine to marginal marine stratigraphic sequences, deposited during global glacioeustatic sea???level cycles corresponding to Marine Isotope Stages (MIS) 78???10. Here, we characterise the sequences using: (1) a series of drill cores sited above and behind the coastal outcrops, which recovered a composite record of c. 450 m, (2) a new high resolution multichannel seismic reflection profile acquired along the beach adjacent to the coastal cliffs, and (3) downhole digital logs from the boreholes. This paper integrates the outcrop and subsurface datasets to produce a high resolution model of the stratigraphic signatures and 2D architecture of a cyclical, shallow???marine deposition system. Such models have significant applications to petroleum exploration, and enable the distribution of reservoir facies and intervening seal rocks within sequences, together with the nature of the connectivity of sandstone facies between sequences, to be evaluated. Similar hydrocarbon???producing systems within the Eocene Kapuni Group (e.g., Mangahewa and Kaimiro formations) have been, and are still, the focus of intense exploration in Taranaki Basin.

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  • A terrestrial record of Last Interglacial climate preserved by voluminous debris avalanche inundation in Taranaki, New Zealand

    Newnham, R; Alloway, Brent (2004-03)

    Journal article
    The University of Auckland Library

    At Airedale Reef, western North Island, New Zealand, a ca. 4???m thick volcanogenic debris avalanche deposit has facilitated the preservation of an enveloping sequence of peats with interbedded andesitic tephras spanning marine isotope (MIS) 5. The sequence closely overlies a wave-cut terrace correlated to MIS 5e and, in turn, is overlain by andic beds with tephra interbeds including the Rotoehu and Kawakawa tephras deposited during early MIS 3 and mid-MIS 2, respectively. Pollen analysis of the organic sequence shows a coherent pattern of fluctuating climate for the Last Interglacial???Last Glacial transition that corresponds with marine isotope stratigraphy and supports the contention that orbital variations were a primary factor in late Quaternary southern mid-latitude climate change. A five-stage subdivision of MIS 5 is clearly recognised, with marine isotope substage (MISS) 5b drier than MISS 5d, and the cooling transition from 5a to MIS 4 also may have been comparatively dry and characterised by natural fire, perhaps associated with volcanism. Several other examples of volcanic impact on vegetation and the landscape are evident. The Airedale Reef sequence exhibits strong similarities with fragmentary MIS 5 pollen records preserved elsewhere in New Zealand and enables the proxy record of southern mid-latitude climatic variability during the Last Interglacial???Glacial cycle to be extended.

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  • Vegetation and climate of Auckland, New Zealand, since ca. 32 000 cal. yr ago: support for an extended LGM

    Newnham, RM; Lowe, DJ; Giles, T; Alloway, Brent (2007-07)

    Journal article
    The University of Auckland Library

    Auckland occupies a climatically sensitive position close to a major biogeographic boundary in the southern mid-latitudes. A new pollen record from Kohuora maar crater, Auckland, displays vegetation and climatic changes for the past ca. 32???000 years. Of particular interest are the inferred climatic patterns for the first part of the interval, encompassing the Last Glacial Maximum (LGM). The Kohuora record corresponds closely with pollen records from other Auckland sites indicating that the patterns observed are at least regional in extent. It is also broadly consistent with a variety of palaeoenvironmental evidence from across New Zealand, including the glacial record from Westland, other palynological records from North Island, other palaeoecological records from the South Island, and aeolian quartz sequences from western North Island. These records show that glacial conditions prevailed across most, if not all, of New Zealand during the interval ca. 29???19???k cal.???yr???BP, longer and earlier than the LGM sensu stricto. We suggest that the term extended LGM (eLGM) may be more appropriate for the New Zealand region. Within this predominantly cold interval, the Auckland pollen records indicate a climatic amelioration for the interval ca. 26???24???k cal. yr BP, also consistent with other palaeocological data from Canterbury, that fall within a period of climate amelioration recognised between the first two eLGM glacial advances in Westland. We refer to this warming interval as the eLGM Interstadial. The ca. 27???k cal. yr BP Kawakawa/Oruanui tephra is instrumental in most of these inter-site comparisons and occurs after the first peak of eLGM cooling in a short-lived comparatively mild phase. A subsequent return to apparently colder climate in the Auckland records may indicate a volcanic cooling effect or, more likely, widespread landscape disturbance following this major eruption event. Strong correspondence between biotic responses, glacial fluctuations and aeolian quartz deposition linked to major shifts in strength and latitudinal extent of the southern westerlies suggest that both the eLGM and eLGM Interstadial may be more widely registered, at least across the Southern Ocean. Support for this assertion comes from parallel investigations in western and southernmost South America and isotopic and palaeoecological records from Southern Ocean marine cores. Recent reconstructions of the globally averaged ice-equivalent sea-level history are in line with this evidence from the Southern Hemisphere, suggesting that the eLGM may have a global registration. In light of these observations, we suggest a re-examination of the defined timing of the LGM along with renewed effort to establish climatic patterns during this period and understand their causes.

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  • Rates of deformation, uplift, and landscape development associated with active folding in the Waipara area of North Canterbury, New Zealand

    Nicol, A; Alloway, Brent; Tonkin, P (1994-12)

    Journal article
    The University of Auckland Library

    Analysis of the geometry and ages of faulted and tilted late Quaternary fluvial terraces and their associated cover beds provide evidence of active folding at three localities in the Waipara area of North Canterbury, New Zealand, Terrace survey data, the occurrence of the approximately 22.6-kyr-old Aokautere Ash, and examination of soil profiles indicate that folding has continued into the late Holocene but that the amounts and rates of deformation are locally variable. Rates of uplift in the Waipara area are compared with those derived from marine terraces preserved at the Pacific coast, east of the study area. Results indicate that rates of measurable deformation reach a maximum along the Waipara range front, where bedrock deformation is most intense and shortening rates of up to 5.57??0.69%/100 kyr occur. Across the coastal ranges the average rate of shortening is 0.8??0.4%/100 kyr, which corresponds with an absolute shortening rate of 1.4??0.6 m/kyr and represents only a small proportion of the predicted plate motion vector in this region. Uplift rates range from 0???1.83 m/kyr for a late last glacial fluvial terrace and from 1.36???2.16 m/kyr for three marine terraces. Fluvial and marine terrace uplift rates vary in accord with the geometries of the folds in bedrock, and the spatial pattern of uplift directly reflects fold growth. The structure contour pattern of folded surfaces provides a first approximation to the spatial pattern of uplift. Differential uplift due to folding accounts for up to approximately 55???75% of the total uplift and has produced folds with structural relief of about 1300 m (i.e., amplitudes of 600???700 m). These folds have formed over the last 0.8??0.4 m.y. since the onset of Quaternary deformation in the Waipara region.

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  • Volcanic-debris-avalanche deposits in New Zealand - Lithofacies organization in unconfined, wet-avalanche flows

    Palmer, BA; Alloway, Brent; Neall, VE (1991)

    Journal article
    The University of Auckland Library

    The lithofacies represent division of the debris-avalanche deposits into megaclast-rich (axial-A) and interclast matrix-rich (axial-B) parts and transformation of the avalanches into lahars (marginal lithofacies). Megaclasts preserving original volcanic stratigraphy in axial-A represent collapse and transport of coherent pieces of the edifice by the debris avalanches. Transformation into lahars occurred as the coarse avalanche debris came to rest and the finer portion continued to flow, enhanced by a secondary component eroded from underlying deposits. In general, flow across relatively featureless ring plains allowed multidirectional-lithofacies development, so that lobes of axial-A are surrounded by axial-B and marginal lithofacies both laterally and, where visible, distally. Variations in avalanche size and ring-plain physiography produced different lithofacies architectures in the New Zealand deposits. Large debris avalanches were little affected by physiography and spread out in fan-shaped sheets comprising near-concentric zones of each lithofacies. Smaller avalanches were confined at some point during flow, so that central parts of the deposit are shoestring diamictons of axial-B and the marginal lithofacies. The tripartite axial-A through axial ?? and marginal lithofacies associations are present in proximal areas of these deposits. Distal areas are usually axial-B and marginal lithofacies.

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  • Vegetation and climate change, fire-regime shifts and volcanic disturbance in Chilo?? Continental (43??S) during the last 10,000 years

    Henr??quez, WI; Moreno, PI; Alloway, Brent; Villarosa, G (2015-09-01)

    Journal article
    The University of Auckland Library

    Disentangling the roles of paleofires and explosive volcanism from climatic drivers of past vegetation change is a subject insufficiently addressed in the paleoecological literature. The coastal region of the Chilo?? Continental sector of northwestern Patagonia is ideal in this regard considering its proximity to active eruptive centers and the possibility of establishing comparisons with more distal, upwind sites where volcanic influence is minimal. Here we present a fine-resolution pollen and macroscopic charcoal record from Lago Teo with the aim of documenting the local vegetation and climate history, and assessing the role of disturbance regimes as drivers of vegetation change during the last ???10,000 years. The Lago Teo record shows a conspicuous warm/dry interval between ???7500 and 10,000 cal yrs BP followed by a cooling trend and increase in precipitation that has persisted until the present, in agreement with previous studies in the region and interpretations of past southern westerly wind activity at multi-millennial scales. The presence of 26 tephras throughout the record allows examination of the relationship between explosive volcanism and vegetation change under contrasting climatic states of the Holocene. We found consistent statistically significant increases in Tepualia stipularis after tephra deposition over the last 10,000 years, in Eucryphia/Caldcluvia between 7500 and 10,000 cal yrs BP and in Hydrangea over the last 7500 years. Our results indicate a primary role of climate change as driver of long-term vegetation change and as a modulator of vegetation responses to volcanic disturbance at multidecadal and centennial timescales.

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  • Correlation and characterisation of individual glass shards from tephra deposits using trace element laser ablation ICP-MS analyses: current status and future potential

    Pearce, NJG; Denton, JS; Perkins, WT; Westgate, JA; Alloway, Brent (2007-10)

    Journal article
    The University of Auckland Library

    Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a high spatial resolution analytical method which has been applied to the analysis of silicic tephras. With current instrumentation, around 30 trace elements can be determined from single glass shards as small as ??? 40?????m, separated from tephra deposits. As a result of element fractionation during the ablation process using a 266???nm laser, a relatively complex calibration strategy is required. Nonetheless, such a strategy gives analyses which are accurate (typically within ??5%) and have an analytical precision which varies from ??? ??2% at 100???ppm, to ??? ??15% at 1???ppm. Detection limits for elements used in correlation and discrimination studies are well below 1???ppm. Examples of the application of trace element analysis by LA-ICP-MS in tephra studies are presented from the USA, New Zealand and the Mediterranean. Improvements in instrumental sensitivity in recent years have the potential to lower detection limits and improve analytical precision, thus allowing the analysis of smaller glass shards from more distal tephras. Laser systems operating at shorter wavelengths (e.g. 193???nm) are now more widely available, and produce a much more controllable ablation in glasses than 266???nm lasers. Crater sizes of <10?????m are easily achieved, and at 193???nm many of the elemental fractionation issues which mar longer wavelengths are overcome. By coupling a short wavelength laser to a modern ICP-MS it should be possible to determine the trace element composition of glass shards as small as 20?????m and, providing sample preparation issues can be overcome, the determination of the more abundant trace elements in glass shards as small as 10?????m is within instrumental capabilities. This will make it possible to chemically fingerprint tephra deposits which are far from their sources, and will greatly extend the range over which geochemical correlation of tephras can be undertaken.

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  • Characterization, identity, distribution, and source of late Cenozoic tephra beds in the Klondike district of the Yukon, Canada

    Preece, SJ; Westgate, JA; Alloway, Brent; Milner, MW (2000-07)

    Journal article
    The University of Auckland Library

    A large number of distal, silicic tephra beds have been preserved in the late Cenozoic deposits of the Klondike region, Yukon Territory. Forty-one tephra samples, representing twelve distinctive beds, are detailed in this study. They range in composition from basaltic andesite to high-silica rhyolite, and were deposited during the late Pliocene to Late Wisconsinan time interval. Seven tephra beds are derived from volcanoes in the Wrangell volcanic field, and four come from the more distant eastern Aleutian arc - Alaska Peninsula region, but the source of the single andesitic tephra is unknown. The widespread and well known Old Crow and Sheep Creek tephra beds have been identified in the Klondike district, but all the other tephra units are characterized in detail for the first time. The ages of most tephra beds are poorly constrained, but will undoubtedly become better known with the application of recently developed glass fission-track methods. Hence, prospects are favourable for the eventual development of a comprehensive and reliable time-stratigraphic framework that will support on-going studies on the late Cenozoic geology, geomorphology, paleontology, and paleoenvironments of the Klondike area.

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  • The AUSTRALASIAN-INTIMATE project special volume

    Barrows, TT; Alloway, Brent; Reeves, J (2013-08-15)

    Journal article
    The University of Auckland Library

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  • Refuting the evidence for an earlier birth of the Taklimakan Desert

    Sun, J; Alloway, Brent; Fang, X; Windley, BF (2015-10-13)

    Journal article
    The University of Auckland Library

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  • An 18,000 year-long eruptive record from Volc??n Chait??n, northwestern Patagonia: Paleoenvironmental and hazard-assessment implications

    Alloway, Brent; Pearce, NJG; Moreno, PI; Villarosa, G; Jara, I; De Pol-Holz, R; Outes, V (2017-07-15)

    Journal article
    The University of Auckland Library

    The 2008 eruption of Volc??n Chait??n (VCha) in northwestern Patagonia was the first explosive rhyolitic eruption to have occurred within a century and provided an unprecedented scientific opportunity to examine all facets of the eruption ranging from magma rheology/ascent rates to ash-fall effects on biota and infrastructure. Up to very recently it was thought that the latest eruption prior to the 2008 event occurred c. 9750 cal. a BP. Although a number of researchers have recognised additional eruptive products, but their stratigraphy, age, and geochemical attributes have not been systematically described and/or recorded. In this study, we provide a detailed examination of andic cover-beds and tephra-bearing lake sequences located both proximally and distally to VCha, which record a series of hitherto unknown rhyolitic eruptive products and place all previous observations firmly within a coherent stratigraphic framework. Through major- and trace-element glass shard geochemistry we are able to confidently verify eruptive source. A total of 20 discrete tephra beds are recognised, with at least 10 having widespread areal distributions and/or depositional imprints broadly comparable to, or greater than, the 2008-tephra event. This record indicates that VCha has been continuously but intermittently active as far back as the end of the Last Glacial Maximum (c. 18,000 cal a BP) with two dominant, genetically related magma types and an intermediary ???mixed??? type. Before this the eruptive record has been largely obscured and/or erased by widespread Andean piedmont glaciation. However, based on the tempo of VCha activity over the last c. 18,000 years, older VCha eruptives can be anticipated to occur as well as future hazardous explosive events. The new eruptive inventory will ultimately be useful for correlating equivalent-aged sequences and refining long-term eruptive tempo as well as corresponding temporal changes in magmatic evolution.

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  • A cosmogenic 3He chronology of late Quaternary glacier fluctuations in North Island, New Zealand (39??S)

    Eaves, SR; Mackintosh, AN; Winckler, G; Schaefer, JM; Alloway, Brent; Townsend, DB (2016-01-15)

    Journal article
    The University of Auckland Library

    Mountain glaciers advance and retreat primarily in response to changes in climate. Establishing the timing and magnitude of mountain glacier fluctuations from geological records can thus help to identify the drivers and mechanisms of past climate change. In this study, we use cosmogenic 3He surface exposure dating and tephrochronology to constrain the timing of past glaciation on Tongariro massif in central North Island, New Zealand (39??S). Exposure ages from moraine boulders show that valley glaciation persisted between c. 30???18 ka, which coincides with the global Last Glacial Maximum. Reinterpretation of moraine tephrostratigraphy, using major element geochemistry analysis, shows that ice retreat and climatic amelioration at the last glacial termination was well underway prior to 14 ka. The equilibrium line altitude in central North Island, during the Last Glacial Maximum, was c. 1400???1550 m above sea level, which is c. 930???1080 m lower than present. Considering the uncertainties in the glacial reconstruction and temperature lapse rates, we estimate that this equilibrium line altitude lowering equates to a temperature depression of 5.6 ?? 1.1 ??C, relative to present. Our mapping and surface exposure dating also show evidence for an earlier period of glaciation, of similar magnitude to the Last Glacial Maximum, which culminated prior to 57 ka, probably during Marine Isotope Stage 4. Good agreement between the timing and magnitude of glacier fluctuations in central North Island and the Southern Alps indicate a response to a common climatic forcing during the last glacial cycle.

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  • Stratigraphy, age and correlation of middle Pleistocene silicic tephras in the Auckland region, New Zealand: A prolific distal record of Taupo Volcanic Zone volcanism

    Alloway, Brent; Westgate, J; Pillans, B; Pearce, N; Newnham, R; Byrami, M; Aarburg, S (2004)

    Journal article
    The University of Auckland Library

    Coastal sections in the Auckland region reveal highly carbonaceous and/or highly weathered clay???dominated cover???bed successions with numerous discrete distal volcanic ash (tephra) layers, fluvially reworked siliciclastic (tephric) deposits, and two widely distributed pyroclastic density current (PDC) deposits generated from explosive silicic volcanism within the Taupo Volcanic Zone (TVZ). The younger of the two PDC deposits (informally named Waiuku tephra) is glass???isothermal plateau fission???track (ITPFT) dated at 1.00 ?? 0.03 Ma and occurs in a normal polarity interval interpreted as the Jaramillo Subchron. Waiuku tephra is correlated with Unit E sourced from the Mangakino Volcanic Centre of the TVZ. Waiuku tephra can be subdivided into two distinctive units enabling unequivocal field correlation: a lower stratified unit (dominantly pyroclastic surge with fall component) and an upper massive to weakly stratified unit (pyroclastic flow). At many sites in south Auckland, Waiuku tephra retains basal ???surge???like??? beds (<1.4 m thickness). This provides clear evidence for primary emplacement and is an exceptional feature considering the c. 200 km this PDC has travelled from its TVZ source area. However, at many other Auckland sites, Waiuku tephra displays transitional sedimentary characteristics indicating lateral transformation from hot, gas???supported flow/surge into water???supported mass flow and hyperconcentrated flow (HCF) deposits. The older PDC deposit is dated at 1.21 ?? 0.09 Ma, is enveloped by tephras that are ITPFT???dated at 1.14 ?? 0.06 Ma (above) and 1.21 ?? 0.06 Ma (below), respectively, and occurs below a short normal polarity interval (Cobb Mountain Subchron) at c. 1.19 Ma. This PDC deposit, correlated with Ongatiti Ignimbrite sourced from the Mangakino Volcanic Centre of TVZ, has laterally transformed from a gas???supported, fine???grained pyroclastic flow deposit at Oruarangi, Port Waikato, into a water???supported volcaniclastic mass flow deposit farther north at Glenbrook Beach. The occurrence of Ongatiti Ignimbrite in Auckland significantly extends its northward distribution. Large numbers of post??? and pre???Ongatiti rhyolitic tephra layers, ranging in age from c. 1.31 to 0.53 Ma, are also recognised in the region, with some up to 0.5 m in compacted fallout thickness. Although some tephras can be attributed to known TVZ eruptions (e.g., Ahuroa/Unit D), many have yet to be identified in proximal source areas and remain uncorrelated. However, some can be reliably correlated to tephra layers occurring in marine to nearshore sequences of Wanganui Basin and deep???sea cores retrieved east of North Island. The identification of previously unrecognised mid???Pleistocene TVZ???sourced tephra deposits in the Auckland region, and their correlation to the offshore marine record, represent an advance in the construction of a higher resolution history for the TVZ where, close to eruptive source, the record is fragmentary and obscured by deep burial, or erosion, or both.

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